1. Field of Invention
The present invention relates to an organic EL (electroluminescent) element which is used in, for example, a display or surface light source device, and to a method of manufacturing the same.
2. Description of Related Art
In recent years, development of an organic EL element (a luminous element having a structure including an anode and a cathode, and a light emitting layer including organic matter sandwiched between the anode and the cathode) has become an increasingly popular way of providing an auto-luminescent type display as an alternative of a liquid crystal display. A conventional structure of the organic EL element includes a structure in which an anode including a transparent electrode, a hole injecting and transporting layer, a light emitting layer, and a cathode which does not transmit light, formed in this order on a transparent substrate. In an organic EL element having this structure, light generated by the combination of an electron and a hole in the light emitting layer is radiated toward the substrate side.
Such an organic EL element has a feature in that high luminance light is highly efficiently emitted by only applying a low voltage. However, the organic EL element has a problem in that this excellent characteristic cannot be obtained when constitutive members of the element deteriorate due to change over time. Causes of this problem particularly include oxidation of the cathode and light emitting layer due to oxygen or water in the air.
Japanese Unexamined Patent Application Publication No. 5-182759 and Japanese Unexamined Patent Application Publication No. 7-282975 attempt to solve this problem by providing the sealing of a laminate with a sealant of a two-layer structure including an inner first sealing layer and an outer second sealing layer, which laminate is formed on a substrate and includes at least an anode, a light emitting layer, and a cathode. These publications mention that a SiO2 film or SiO film is formed as the first sealing layer by a sputtering process or vapor deposition process, and a moisture proof photo-curable resinous layer or thermoplastic polymer layer is formed as the second sealing layer. On the other hand, when the cathode transmits light as well as the anode, light generated by the combination of an electron and a hole in the light emitting layer is also radiated toward the cathode side. Page 2606 of xe2x80x9cAppl. Phys. Lett. Vol. 68(19), May 6, 1996xe2x80x9d describes an organic EL element, in which a thin film which is obtained by vapor co-deposition of Mg and Ag is formed as the cathode and light is therefore also radiated toward the cathode side. In this organic EL element, quinolinolatoaluminum complex (Alq3), an organic material having a low molecular weight, is used as the light emitting layer. Additionally, an ITO film is formed on the cathode including Mg and Ag by a sputtering process. This ITO film is provided as a sealing layer and auxiliary electrode.
Additionally, this publication discloses that the sealing method of the cathode side has a significant influence on initial characteristics. Specifically, the publication mentions that the threshold voltage of the organic EL element is increased, resulting from the formation conditions (e.g., amount of oxygen) of the sealing layer.
However, when a SiO2 film or SiO film is formed as a first sealing layer in the sealing operation with a sealant having a two-layer structure, there is a possibility of oxidation of the cathode which comes in contact with the first sealing layer. Even in the organic EL element in which light is radiated toward both electrode sides, a material having a low work function such as calcium is preferably used as the cathode. In this case, oxidation of the cathode is particularly liable to occur when a SiO2 film or SiO film is formed as the first sealing layer.
The present invention solves these problems of the conventional technologies, by satisfactorily sealing an organic EL element even when light is radiated toward both electrode sides in the organic EL element.
To solve the above problem, the present invention provides an organic EL element which includes a substrate and a laminate formed on the substrate, the laminate having a structure including a cathode, an anode, and a light emitting layer, the light emitting layer including an organic material and being sandwiched between the cathode and anode, and at least the surface of the laminate on the opposite side to the substrate side being sealed by a sealant having a two-layer structure including an inner first sealing layer and an outer second sealing layer. In the organic EL element, the first sealing layer includes a halide of an alkali metal or alkaline earth metal, and the second sealing layer includes a moisture-proof resinous material.
In this organic EL element, the first sealing layer includes a halide of an alkali metal or alkaline earth metal, and at least the surface of the laminate on the opposite side to the substrate side becomes resistant to oxidation.
The halide of alkali metal or alkaline earth metal constituting the first sealing layer includes, for example, lithium fluoride (LiF), calcium fluoride (CaF2), sodium fluoride (NaF), sodium chloride (NaCl), potassium chloride (KCl), potassium bromide (KBr), magnesium fluoride (MgF2), and lithium chloride (LiCl). Among these, lithium fluoride (LiF) is particularly preferred.
In particular, even when an electrode arranged in the laminate on the opposite side to the substrate side is formed by a metallic material, which has a low work function and is very liable to be oxidized (e.g., a material containing calcium), oxidation of the electrode can be effectively inhibited by forming the first sealing layer to be in contact with this electrode as a layer including a halide of an alkali metal or alkaline earth metal (particularly LiF).
Additionally, a layer including LiF can have a transparency in visible light region higher than that of glass, by forming the layer to an appropriate thickness. Accordingly, this layer is suitable as a first sealant for use in the organic EL element in which light is radiated toward both electrode sides.
Materials for the second sealing layer include, for example, an acrylic resin and an epoxy resin. Among these, the epoxy resin is particularly preferred since it is highly resistant to moisture and has a high transmittance of visible light. Such a second sealing layer including an epoxy resin can be easily formed by, for example, applying a liquid thermosetting epoxy resin or light-curable epoxy resin and curing the applied resin.
In order to inhibit deterioration of the laminate (particularly of the light emitting layer) in the formation of the second sealing layer, as well as to obtain the sufficient inhibitory effect of the oxidation of electrode, the first sealing layer should preferably have a thickness of not less than 300 angstroms. Furthermore, the first sealing layer should preferably have a thickness of not more than 500 angstroms, from the viewpoint of inhibiting deterioration of the light emitting layer or the like due to heat in the film-formation of the first sealing layer.
Furthermore, in the organic EL element of the invention, it is preferred that the face on the opposite side to the substrate side and the end face of the laminate are sealed by a sealant having a two-layer structure, which two-layer structure includes a first sealing layer including a halide of an alkali metal or alkaline earth metal, and a second sealing layer including a moisture proof resinous material.
The present invention provides, in another aspect, a method of manufacturing an organic EL element, which includes the steps of forming a laminate on a substrate, said laminate having a structure including a cathode, an anode, and a light emitting layer, said light emitting layer including an organic material and being sandwiched between the cathode and anode; forming a first sealing layer at least on the surface of the formed laminate on the opposite side to the substrate side in an atmosphere substantially free from oxygen and water, said first sealing layer including a halide of an alkali metal or alkaline earth metal; and forming a second sealing layer on the outer side of the first sealing layer in an atmosphere substantially free from oxygen and water, said second sealing layer including a moisture proof resinous material.
In the method of the invention, a layer including LiF is preferably formed as the first sealing layer. In the invented method, a layer includes an epoxy resin is preferably formed as the second sealing layer.
When the first sealing layer including LiF is formed by vacuum deposition process, the formation is preferably performed in the condition that a film-forming rate is 8 angstroms or more per 1 second, from the viewpoint of inhibiting deterioration of the light emitting layer or the like due to heat in the film-formation of the first sealing layer.
In the method of the invention, the first sealing layer is preferably formed to a thickness of not less than 300 angstroms.
It is preferable in the method of the invention to form the first sealing layer including a halide of an alkali metal or alkaline earth metal and to form the second sealing layer including a moisture proof resinous material on the end face of the laminate, as well as the face of the laminate on the opposite side to the substrate side.